The purpose of this research is to contribute to our understanding of the neuronal machinery responsible for learning and response selection in complex integrated behavioral repertoires. These related problems are to be studied in a relatively simple system, the crayfish escape response repertoire, which consists of a small set of escape responses all of which are subject to habituation and probably also to other forms of plastic modification. Specific behavioral, electrophysiological, and anatomical research is proposed on (1) cellular mechanisms of short and long-term habituation, (2) a variety of tropic and developmental phenomena whose underlying mechanisms could also be involved in plastic change due to experience (i.e., "learning"), and (3) the modulatory superstructure that is involved in the integration of escape into the overall behavior of the animal.